I can tell you that you're either using Windows Vista or Windows XP.
I use my brother's Alienware computer over the weekend, if I remember correctly, it has a Windows 7 OS(You can't really tell because of an ass-ton of customization)
His computer still does it, it's something to do with Windows Vista and Windows 7 changing the theme when you play a game. Then when you Alt+Tab out of the game, it freezes up so the computer can change the theme.
You should be able to find where to change a few settings and fix the issue, I would start with compatibility though.